Phased arrays have included multiple antennas coupled with analog phase shifters that allow electromagnetic energy, such as radio frequency (RF) signals, to be sent and received along desired wave-front directions. The effective directivity pattern, or beam shape, of an array of antenna elements can be changed by altering the relative phase of the coherent RF energy arriving at, or emitted from, each element. For example, if all of the elements in a plane of equally spaced identical elements are fed by the same RF signal, the intensity of the radiated electromagnetic energy will be greatest along a line perpendicular to this plane. Alternatively, if the elements are each fed with a progressively phase-shifted RF signal, the direction of maximum radiated intensity will be at some angle away from the normal, or broadside, direction. An antenna system wherein the beam direction of an array of elements can be steered using electronic shifting of the relative element phase is often called an electronically steered antenna (ESA).
Many different kinds of phase shifters are available for controlling the relative phase of RF energy feeding antenna element arrays. These can include ferrites, diode-switched delay lines, and micro-electromechanical switches (MEMS). All of these technologies can be arranged to provide a digitally programmable phase delay (or shift) to each element by using a digitally weighted control signal to adjust the phase properties of the element. However, since these phase shifter circuits must be placed in the analog RF signal path that is between the energy source (e.g., the transmitter) and the antenna elements, it is always the case that some RF energy is lost to dissipation and radiation within the phase shifter. A typical phase shifter, for example, might introduce 0.5 dB of insertion loss per bit of phase shift control. A 5-bit phase shifter, typical of many radar systems, would thereby incur a minimum 2.5 dB insertion loss using such a device. Insertion losses require higher transmitter powers to achieve a given radiated power from the antenna elements.
When used in the receive path of a transmit/receive phased array system, phase shifter loss degrades the sensitivity and noise figure of the phased array receiver. This in turn requires high amplifier gain and results in a reduction of useable bandwidth. Moreover, many phase shifters must trade insertion loss for useable bandwidth. For example, a phase shifter useful over the X-band (8–12 GHz) might be excessively lossy if it must in addition be made to operate from 2–30 GHz. The phase shifting properties of a low-loss phase shifter will almost always be frequency dependent so that wide bandwidth signals, such as radar pulses, may undergo phase distortion as they pass through a phase shifter.
In short, although programmable phase shifters are useful for electronically steering phased array antennas, they are problematic when they must either satisfy the requirement of low loss or wide-bandwidth operation.
Prior attempts to minimize the negative impact of phase shifters on phased array performance involve the development of lower loss and broader bandwidth phase shifter technology. One such technology focuses on using micro-electromechanical switches (MEMS) for the phase shifter circuitry. MEMS devices utilize electrically controlled mechanical switches that require less power than other types of phase shifters. Because it is cumbersome to use MEMS phase shifters to control the entire phase shift in large arrays, secondary phase shifters are usually used to phase-combine multiple sections of the array until a single signal channel is obtained. This combining technique thereby reduces the range of phase shift required by the MEMS phase shifter devices. Because the signal level and signal-to-noise ratio can be degraded by the series combination of many layers of analog phase shifters, this combining technique requires the additional use of many broadband amplifiers to re-generate the signal as it progresses through the combined network.
Recently, the phased array industry has also proposed to construct phased array receivers and transmitters wherein analog-to-digital and/or digital-to-analog circuitry is associated with each antenna element. For example, in the receive mode, a separate analog-to-digital converter (ADC) is used to digitize the RF signal collected by each separate element, and these many digital data streams are then electronically combined to provide a single signal characteristic of the many signals collected by the entire antenna. In transmit mode, a digital-to-analog converter (DAC) is placed at each antenna element such that a digital data stream can be converted by this DAC into an analog RF signal. The generated RF signal from each DAC is then amplified and fed to its associated antenna element. Due to the all-digital interface between the antenna element and the rest of the phased array system, this phased array concept may be considered a “digital” antenna.
FIG. 1 (Prior Art) depicts an example embodiment for such a digital phased array circuitry. An antenna 140 is connected to a switch 136 that in turn connects either the receive path signal 134 or the transmit path signal 132 to the output line 138. Looking to the receive path, the receive path signal 134 connects to a low noise amplifier (LNA) 114, then to a phase shifter 102, and ultimately to an analog-to-digital converter (ADC) 108. ADC 108 provides an M-bit receive data signal 128 that may used by further beam-forming circuitry. Looking to the transmit path, a digital-to-analog converter (DAC) 112 receives an M-bit transmit data signal 130 and provides an analog signal to a phase shifter 104. The output of the phase shifter 104 connects to a power amplifier (PA) 116 and then to the transmit path signal 132. The ADC 108 and the DAC 112 have sampling rates controlled by clock signals (SCLK) 124 and 126 provided by clock circuitry 110.
The phase shifters 102 and 104 add a programmable delay to their relative analog input signals. Thus, phase shifter 102 delays signal 142 with respect to signal 140 by a programmed amount, and phase shifter 104 delays signal 144 with respect to signal 146 by a programmed amount. The amount of the delay is determined by the control register 106. Based upon the delay value 118 provided to the control register 106, the control register 106 provides phase shifters 102 and 104 with X-bit digital control words 120 and 122, respectively. These control words 120 and 122 determine the amount of the delay added to the analog signals passing through the phase shifters 102 and 104.
Although the antenna embodiment of FIG. 1 (prior art) may provide a digital interface, it still requires analog phase shifters between the antenna element and the ADC or DAC to provide the fine phase shifting function needed to fine-steer the overall antenna pattern. In other words, the inclusion of an ADC or DAC near the antenna element does not mitigate the negative impact of phase shifters on array performance.